1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to large relative aperture, high range zoom lenses for use in photographic cameras, video cameras or broadcasting cameras. Still more particularly, it relates to zoom lenses whose zoom ratio is 12 and F-number is about 1.8 to 2.2 and which nonetheless are of the wide-angle type with a field angle of 70 degrees or wider at the wide-angle end.
2. Description of Related Art
In photographic cameras or video cameras, many zoom lenses of the so-called "rear focus" type have been proposed in which a lens unit other than the front or first lens unit is made movable for focusing.
In general, as compared with the other type in which the first lens unit is moved to effect focusing, the rear focus type of zoom lens has such advantages that the effective diameter of the first lens unit becomes smaller, thereby minimizing the bulk and size of the entirety of the lens system, that close-up photography, particularly, supershort focusing, is carried out with ease, and further that, since the lens unit for focusing is smaller in size and lighter in weight, because a weaker driving torque suffices for moving it, rapid focus adjustment can be carried out.
In Japanese Laid-Open Patent Applications Nos. Sho 62-247316 and Sho 62-24213, a zoom lens is disclosed comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totalling four lens units, wherein the second lens unit is made axially movable for varying the focal length and the compensation for the image shift with zooming and the focusing are carried out by moving the fourth lens unit.
Japanese Laid-Open Patent Applications No. Hei 2-39011 discloses a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power axially movable for varying the focal length, a third lens unit of positive refractive power for condensing light and a fourth lens unit of positive refractive power axially movable for keeping the image plane at a constant position from a reference plane, which would otherwise shift when the second lens unit moves or when the object distance varies. The first lens unit is constructed with a single lens of negative refractive power and a single lens of positive refractive power. The zoom lens employs an aspheric surface.
Japanese Laid-Open Patent Application No. Hei 3-180809 discloses a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units. During zooming, the second lens unit moves, while at least one of the other lens units moves to compensate for the image shift. The first lens unit is constructed with a negative lens of meniscus form convex toward the object side and, behind an appreciable space, a positive lens having a strong refracting surface facing the object side, totaling two lenses.
Japanese Laid-Open Patent Application No. Hei 6-324265 discloses a zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second and fourth lens units being axially moved so that the air separation between any adjacent two of the lens units varies to continuously vary the focal length. The first lens unit is constructed with four lenses, i.e., a positive lens, a negative lens, a positive lens and a positive lens in this order from the object side, thus contributing to an increase of the angle of field.
In general, the use of the rear focus type in the zoom lens leads to minimize the bulk and size of the lens system as a whole. Also, rapid focusing becomes possible. On the other hand, however, during focusing, the aberrations vary to a greater extent, giving rise to a very difficult problem of simultaneously fulfilling the requirements of achieving improvements of the compact form and of maintaining a good stability of high optical performance throughout the entire focusing range. Particularly, in the large relative aperture, high range zoom lens, the problem of obtaining a high optical performance throughout the entire zooming range as well as the entire focusing range becomes more serious.
To achieve simultaneous increases of the angular field and the zooming range, the front or first lens unit has to increase in the effective diameter. So, another problem arises in that the lens system as a whole becomes longer and bulkier.
For example, in the above-mentioned Japanese Laid-Open Patent Application No. Hei 2-39011, the proposed zoom lens has its angle of field being not sufficient. Another proposed zoom lens in the above-mentioned Japanese Laid-Open Patent Application No. Hei 3-180809 makes use of an air lens in the first lens unit for the purpose of correcting aberrations, but its angle of field is not necessarily wide enough. Also, the wide-angle zoom lens proposed in the above-mentioned Japanese Laid-Open Patent Application No. Hei 6-324265 has its first lens unit constructed, in an actual practice of its embodiment, from a negative lens, a positive lens, a positive lens and a positive lens. However, because of the narrow air separation between the negative lens and the positive lens, the rear principal point of the first lens unit is not brought sufficiently close to the second lens unit. Therefore, the diameter of the front lens members becomes greater than might be expected from the angle of field.